In a so-called force control operation, the position of the tip end (such as the end position of an arm) of a robot in the task coordinate system and the contact force of the tip end are controlled while the tip end is in contact with an object. In such a force control operation, it is essential to estimate the external force acting on the tip end, and flexibly perform an operation correction in accordance with the estimated external force. In a case where a force sensor is attached to the tip end, the external force can be measured with high accuracy. However, a force sensor is expensive and is easily damaged by shocks. Therefore, the use of a force sensor is often avoided. Where no force sensors are used, it is necessary to estimate an actuator disturbance torque generated by the external force acting on the robot. The disturbance torque is calculated by subtracting the estimated drive torque necessary for movement of the actuator from an actually drive torque command value. Therefore, it is necessary to identify, with high accuracy, the parameters of the dynamic model to be used for the calculation.
Particularly, it is known that a friction varies in a complicated manner due to short-term factors such as the actuator speed, the operating direction, the operating history, and the variations of loads attached to the tip end, as well as long-term factors such as temperature variations over different seasons and different times of the day and age-related degradation depending on the state of use. Since the parameters contained in a friction approximation model vary due to the above-mentioned factors, it is necessary to identify appropriate parameters in real time. There has been a known technique of identifying a coulomb friction coefficient, a viscous friction coefficient, and a constant disturbance torque in real time.
According to the technique of identifying a coulomb friction coefficient, a viscous friction coefficient, and a constant disturbance torque in real time, the viscous friction is approximated by a broken line. However, it is a known fact that a friction torque gradually becomes saturated as the drive speed becomes higher. To perform high-accuracy approximation at various speeds in a wider range, approximation models such as an arc tangent or a linear sum of exponential functions have been suggested. With those models, it is also necessary to identify parameters in real time.